Microcystis aeruginosa produces toxins known as microcystins. These potent toxins, inhibitors of protein phosphates, have been shown to cause hepatocellular damage, resulting in massive liver necrosis and acute death in animals. Human exposure to these toxins through drinking water has been shown to cause elevation of liver enzymes, indicating liver damage. Recently, interest has arisen concerning other possible outcomes of cell exposure to microcystins. First, microcystins have been reported to cause apoptosis, a form of "programmed cell death", distinct from necrosis. Investigators have found that microcystins can produce many of the morphologic changes associated with apoptosis. Additional cellular and molecular studies to confirm the ability of the toxin to cause apoptosis have not been accomplished. A primary goal of this proposed research is to confirm or reject (using molecular and cellular techniques) the hypothesis that microcystins may induce apoptosis in cells. DNA fragmentation, morphologic changes characteristic of apoptosis, and presence of the c-fos product will be investigated in a variety of cell types exposed to microcystins. In addition, the role of tumor necrosis factor in initiation of apoptosis in cells will be investigated. Second, microcystins have been shown to increase tumor formation in laboratory animals and to be linked to increased incidence of hepatocellular carcinoma in humans. Little work has been accomplished to explain the mechanisms through which this toxin acts. This proposed research will investigate the ability of microcystins to induce tumor formation in a variety of cell types, evaluate concentrations of microcystins, cell types exposed, and time of exposure to attempt to identify conditions that select for one outcome of microcystins toxicity over another. Increasing our knowledge of a ubiquitous toxin, such as microcystins, has the potential to directly prevent intoxication disease in humans and animals. It also has the potential to indirectly prevent or treat other disease conditions, by using microcystins as a tool to study cell cycle events.